Solvent pervaporation through a membrane is a well known phenomenon that has been harnessed in membrane separation applications. For example, the prior art is rich with examples of the use of solvent pervaporation through a membrane for the purpose of concentrating relatively low vapor pressure components on a retentate side of the membrane. In addition, distillation operations utilizing pervaporation through a membrane have been performed to selectively recover solvent components on the permeate side of the membrane.
While the beneficial aspects of pervaporation have long been known and utilized in purposeful solvent separation processes, such pervaporation characteristics can have significant negative effects in mixed-solvent applications wherein the relative concentrations of the respective solvents is desired to be known and/or constant. A particular example of such a mixed-solvent application is in liquid chromatography systems, wherein mobile phases made up of more than one solvent are used. It has been recognized by the Applicants, however, that changes to the relative concentrations of the mobile phases can occur over time, thereby negatively effecting the accuracy of chromatographic analysis.
Pervaporation effects are particularly damaging to analytical accuracy in chromatographic systems utilizing relatively low through-put mobile phase volumes, or in instances wherein the chromatographic instrumentation is only periodically operated without complete flushing of supply lines between each operation. For example, systems that utilize mobile phase flow rates of on the order of nanoliters or microliters per hour are at risk of having the relative concentrations of the solvents making up the mobile phase being substantially modified during analyte transportation through the chromatographic instrumentation.
In particular, liquid chromatography systems typically employ degassing chambers in which the liquid mobile phase is exposed to a degassing environment through a gas-permeable, liquid-impermeable membrane. Such a degassing environment may be, for example, relatively low absolute pressures maintained by evacuation pumps, or relatively low target material partial pressures in a sweep fluid passed through a permeate side of a degassing chamber. Typically, degassing operations have been arranged and controlled to maximize degassing performance on the mobile phase passing through the degassing chamber. To do so, vacuum pumps were programmed to maintain very low absolute pressures on the permeate side of the membrane, or, in cases of a sweep fluid, the sweep fluid utilized contained little or no concentration of the targeted gas species being withdrawn from the mobile phase. In both cases, a target gas concentration differential has traditionally maintained a relatively high value to drive target gas transfer through the membrane to the permeate side. A result of maintaining such a large target gas concentration differential at all times in the degassing chamber is the causation of pervaporative effects. Specifically, relatively long residence time of mobile phase within the degassing chamber having a permeate side maintained at the conditions described above has the tendency to cause mobile phase component materials having relatively high vapor pressures and high membrane solubility to pervaporate through the membrane, while relatively lower vapor pressure mobile phase component materials have a lower tendency, and thus a lower rate, of pervaporation through the membrane. As a consequence, the mobile phase on the retentate side of the degassing chamber can become concentrated in relatively higher vapor pressure component materials, particularly if such mobile phase has a relatively high residence time within the degassing chamber.
It is therefore a principal object of the present invention to provide a control mechanism for maintaining the environment on the permeate side of a membrane at one or more conditions effective in limiting pervaporation through the membrane of a mobile phase having two or more component materials.
It is another object of the present invention to provide a method for controlling pervaporation of a mobile phase having two or more component materials through a membrane.
It is a further object of the present invention to provide a method and apparatus for maintaining the partial pressures of selected ones of the mobile phase component materials on a permeate side of a separation membrane substantially equal to the respective vapor pressures of the selected mobile phase component materials, with such an environment defining a first condition that is maintained on the permeate side of the chamber only during designated time periods.